CS448f: Image Processing For Photography and Vision Blending and Pyramids

Blending We’ve aligned our images. What now? Averaging Weighted averaging min/max/median

Noise reduction by Averaging

2 Shots

4 Shots

8 Shots

16 Shots

Noise Reduction by Averaging We’re averaging random variables X and Y Both have variance S 2 Variance of X+Y = 2S 2 Std.Dev. of X+Y = sqrt(2). S Std.Dev of (X+Y)/2 = sqrt(2)/2. S Ie, every time we take twice as many photos, we reduce noise by sqrt(2)

Noise Reduction by Averaging Average 4 photos: noise gets reduced 2x Average 8 photos: noise gets reduced 3x Average 16 photos: noise gets reduced 4x

Noise Reduction by Median (demo)

Median v Average

Can we identify the bad pixels? They’re unlike their neighbours Instead of averaging, weighted average – where weight = similarity to neighbours

Weighted Average

Can we identify the bad pixels? They’re unlike their neighbours Instead of averaging, weighted average – where weight = similarity to neighbours Favors blurriness 

Input

Other uses of Median Removing Transient Occluders (live demo) (Gates demo) (surf demo)

Panorama Stitching

Multiple Exposure Fusion

Focus Fusion

Pyramids We’ve been breaking images into two terms for a variety of apps – Coarse + Fine More generally we can break it into many terms: – Very coarse + finer + finer... + finest.

Pyramids We can do this by blurring more and more:

Pyramids And then (optionally) taking differences --

Pyramids The coarse layers can be stored at low res. Gaussian Pyramid Laplacian Pyramid

Pyramids How much memory does this use?

Pyramid Uses: Sampling arbitrarily sized Gaussians Equalizing an image – The different levels represent different frequency ranges – We can scale each frequency level and recombine Blending multiple images

Pyramid Blending Key Insight: – Coarse structure should blend very slowly between images (lots of feathering), while fine details should transition more quickly. More robust to tricky cases than plain old compositing

Inputs:

Compositing: Hard Mask

Compositing: Soft Mask

Multi-Band Blending

Exposure Fusion